Flow batteries having an electrode with differing hydrophilicity on opposing faces and methods for production and use thereof

ABSTRACT

Electrochemical cells, such as those present within flow batteries, can include at least one electrode with one face being more hydrophilic than is the other. Such electrodes can lessen the incidence of parasitic reactions by directing convective electrolyte circulation toward a separator in the electrochemical cell. Flow batteries containing the electrochemical cells can include: a first half-cell containing a first electrode with a first face and a second face that are directionally opposite one another, a second half-cell containing a second electrode with a first face and a second face that are directionally opposite one another, and a separator disposed between the first half-cell and the second half-cell. The first face of both the first and second electrodes is disposed adjacent to the separator. The first face of at least one of the first electrode and the second electrode is more hydrophilic than is the second face.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to energy storage and, morespecifically, to modifications and techniques for improving theperformance of flow batteries and related electrochemical systems.

BACKGROUND

Electrochemical energy storage systems, such as batteries,supercapacitors and the like, have been widely proposed for large-scaleenergy storage applications. Various battery designs, including flowbatteries, have been considered for this purpose. Compared to othertypes of electrochemical energy storage systems, flow batteries can beadvantageous, particularly for large-scale applications, due to theirability to decouple the parameters of power density and energy densityfrom one another.

Flow batteries generally include negative and positive active materialsin corresponding electrolyte solutions, which are flowed separatelyacross opposing faces of a membrane or separator in an electrochemicalcell containing negative and positive electrodes. The terms “membrane”and “separator” are used synonymously herein. The flow battery ischarged or discharged through electrochemical reactions of the activematerials that occur inside the two half-cells. As used herein, theterms “active material,” “electroactive material,” “redox-activematerial” or variants thereof synonymously refer to materials thatundergo a change in oxidation state during operation of a flow batteryor like electrochemical energy storage system (i.e., during charging ordischarging).

Although flow batteries hold significant promise for large-scale energystorage applications, they have historically been plagued by sub-optimalenergy storage performance (e.g., round trip energy efficiency) andlimited cycle life, among other factors. Certain factors leading tosub-optimal performance are discussed hereinafter. Despite significantinvestigational efforts, no commercially viable flow batterytechnologies have yet been developed.

Balanced oxidation and reduction of the active materials in a flowbattery are desirable electrochemical reactions, since they contributeto the battery's proper operation during charging and dischargingcycles. Such reactions may be referred to herein as “productivereactions.”

In addition to desirable productive reactions, undesirable parasiticreactions can also occur within one or both half-cells of flow batteriesand related electrochemical systems. As used herein, the term “parasiticreaction” refers to any side electrochemical reaction that takes placein conjunction with productive reactions. Parasitic reactions can ofteninvolve a component of an electrolyte solution that is not the activematerial. Electrochemical reactions of an active material that renderthe active material unable to undergo reversible oxidation and reductioncan also be considered parasitic in nature. Parasitic reactions that cancommonly occur in electrochemical cells containing an aqueouselectrolyte solution are evolution of hydrogen and/or oxidation byoxygen. Hydrogen evolution, for example, can at least partiallydischarge the negative electrolyte solution of an electrochemical cell.Related parasitic reactions can also occur in non-aqueous electrolytesolutions.

Discharge associated with parasitic reactions can decrease the operatingefficiency and other performance parameters of a flow battery. Inaddition, parasitic reactions can change the pH of an electrolytesolution, which can destabilize the active material therein in somecases. In the case of a parasitic reaction that occurs preferentially inone half-cell over the other, an imbalance in state of charge can resultbetween the negative and positive electrolyte solutions. The term “stateof charge” (SOC) is a well understood electrochemical energy storageterm that refers herein to the relative amounts of reduced and oxidizedspecies at an electrode within a given half-cell of an electrochemicalsystem. Charge imbalance between the electrolyte solutions of a flowbattery can lead to mass transport limitations at one or both of theelectrodes, thereby lowering the round-trip operating efficiency. Sincethe charge imbalance can be additive with each completed charge anddischarge cycle, increasingly diminished performance of a flow batterycan result due to parasitic reactions.

Charge rebalancing of one or both electrolyte solutions can be conductedto combat the effects of parasitic reactions. Although various chargerebalancing techniques are available, they can be costly andtime-consuming to implement. Determining the true concentration ofoxidized and reduced active material species in an electrolyte solutioncan oftentimes itself be difficult, thereby adding a further difficultyto the charge rebalancing process. While charge rebalancing of anelectrolyte solution can often be accomplished given sufficientdiligence, the accompanying pH changes can frequently be much moredifficult to address.

In view of the foregoing, flow batteries and other electrochemicalsystems configured to decrease the incidence of parasitic reactions andother performance-reducing factors would be highly desirable in the art.The present disclosure satisfies the foregoing needs and providesrelated advantages as well.

SUMMARY

In some embodiments, the present disclosure provides flow batteriesincluding a first half-cell containing a first electrode with a firstface and a second face that are directionally opposite one another, asecond half-cell containing a second electrode with a first face and asecond face that are directionally opposite one another, and a separatordisposed between the first half-cell and the second half-cell. The firstface of both the first and second electrodes is disposed adjacent to theseparator. The first face of at least one of the first electrode and thesecond electrode is more hydrophilic than is the second face.

In other various embodiments, the present disclosure provides methodsfor fabricating an electrochemical cell having at least one electrodewith gradient hydrophilicity. The methods can include providing aconductive material with a first face and a second face that aredirectionally opposite one another, in which the first face is morehydrophilic than is the second face; and forming an electrochemical cellincluding a first half-cell containing a first electrode, a secondhalf-cell containing a second electrode, and a separator disposedbetween the first half-cell and the second half-cell. At least one ofthe first electrode and the second electrode includes the conductivematerial, and the first face of the conductive material is disposedadjacent to the separator.

In still other various embodiments, the present disclosure describesmethods for operating a flow battery having an electrode with gradienthydrophilicity. The methods can include providing a flow batteryincluding an electrochemical cell containing: a first half-cellcontaining a first electrode with a first face and a second face thatare directionally opposite one another, a second half-cell containing asecond electrode with a first face and a second face that aredirectionally opposite one another, and a separator disposed between thefirst half-cell and the second half-cell, in which the first face ofboth the first and second electrodes is disposed adjacent to theseparator, and the first face of at least one of the first electrode andthe second electrode is more hydrophilic than is the second face; andcirculating a first electrolyte solution through the first half-cell anda second electrolyte solution through the second half-cell. Convectiveflow of at least one of the first electrolyte solution and the secondelectrolyte solution occurs preferentially in a hydrophilic region ofthe first electrode or the second electrode proximate the separator.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter. These and other advantages and featureswill become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 shows a schematic of an illustrative flow battery containing asingle electrochemical cell;

FIG. 2 shows a schematic of an illustrative electrochemical cellconfiguration containing a bipolar plate abutting each electrode;

FIG. 3 shows an illustrative schematic of a bipolar plate containinginterdigitated flow channels;

FIG. 4 shows a schematic of an illustrative electrochemical cellconfiguration in which the relative hydrophilicity of the electrodes hasbeen increased proximate the separator;

FIGS. 5A and 5B show illustrative schematics demonstrating differencesin fluid flow dynamics in the absence and in the presence of anelectrode having a hydrophilicity gradient;

FIG. 6 shows a comparative plot of calculated cell resistance for ahomogenous electrode and an engineered gradient electrode as a functionof state of charge;

FIGS. 7 and 8 show illustrative schematics demonstrating how surfaceoxides in a carbon material can be manipulated to alter thehydrophilicity of one electrode face to a greater extent than the other;and

FIG. 9 shows an illustrative schematic demonstrating how plasmafunctionalization can be used to increase the hydrophilicity of oneelectrode face to a greater extent than the other.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to flow batteriescontaining at least one electrode with one face that is more hydrophilicthan is the other, thereby leading to gradient hydrophilicity in the atleast one electrode. The present disclosure is also directed, in part,to methods for fabricating an electrochemical cell containing at leastone electrode with gradient hydrophilicity. The present disclosure isalso directed, in part, to methods for improving the operatingperformance of electrochemical cells within flow batteries and relatedelectrochemical systems by utilizing one or more electrodes havinggradient hydrophilicity.

The present disclosure may be understood more readily by reference tothe following description taken in connection with the accompanyingfigures and examples, all of which form a part of this disclosure. It isto be understood that this disclosure is not limited to the specificproducts, methods, conditions or parameters described and/or shownherein. Further, the terminology used herein is for purposes ofdescribing particular embodiments by way of example only and is notintended to be limiting unless otherwise specified. Similarly, unlessspecifically stated otherwise, any description herein directed to acomposition is intended to refer to both solid and liquid versions ofthe composition, including solutions and electrolytes containing thecomposition, and electrochemical cells, flow batteries, and other energystorage systems containing such solutions and electrolytes. Further, itis to be recognized that where the disclosure herein describes anelectrochemical cell, flow battery, or other energy storage system, itis to be appreciated that methods for operating the electrochemicalcell, flow battery, or other energy storage system are also implicitlydescribed.

It is also to be appreciated that certain features of the presentdisclosure may be described herein in the context of separateembodiments for clarity purposes, but may also be provided incombination with one another in a single embodiment. That is, unlessobviously incompatible or specifically excluded, each individualembodiment is deemed to be combinable with any other embodiment(s) andthe combination is considered to represent another distinct embodiment.Conversely, various features of the present disclosure that aredescribed in the context of a single embodiment for brevity's sake mayalso be provided separately or in any sub-combination. Finally, while aparticular embodiment may be described as part of a series of steps orpart of a more general structure, each step or sub-structure may also beconsidered an independent embodiment in itself.

Unless stated otherwise, it is to be understood that each individualelement in a list and every combination of individual elements in thatlist is to be interpreted as a distinct embodiment. For example, a listof embodiments presented as “A, B, or C” is to be interpreted asincluding the embodiments “A,” “B,” “C,” “A or B,” “A or C,” “B or C,”or “A, B, or C.”

In the present disclosure, the singular forms of the articles “a,” “an,”and “the” also include the corresponding plural references, andreference to a particular numerical value includes at least thatparticular value, unless the context clearly indicates otherwise. Thus,for example, reference to “a material” is a reference to at least one ofsuch materials and equivalents thereof.

In general, use of the term “about” indicates approximations that canvary depending on the desired properties sought to be obtained by thedisclosed subject matter and is to be interpreted in a context-dependentmanner based on functionality. Accordingly, one having ordinary skill inthe art will be able to interpret a degree of variance on a case-by-casebasis. In some instances, the number of significant figures used whenexpressing a particular value may be a representative technique ofdetermining the variance permitted by the term “about.” In other cases,the gradations in a series of values may be used to determine the rangeof variance permitted by the term “about.” Further, all ranges in thepresent disclosure are inclusive and combinable, and references tovalues stated in ranges include every value within that range.

As discussed above, energy storage systems that are operable on a largescale while maintaining high efficiency values can be extremelydesirable. Flow batteries have generated significant interest in thisregard, but there remains considerable room for improving theiroperating characteristics. In particular, parasitic reactions cansignificantly impact the operating efficiency in conventional flowbattery designs. At the very least, parasitic reactions can create animbalance in state of charge between the two electrolyte solutions of aflow battery, which can impact the battery's performance over time andrepeated cycles of charging and discharging. In addition, parasiticreactions can lead to undesirable changes in the pH of one or more ofthe electrolyte solutions, which can affect stability the activematerials in some cases. In conventional flow battery designs, parasiticreactions can be problematic to manage due to a number of operationaldifficulties.

Given the issues associated with parasitic reactions and otherperformance-reducing factors, the present inventors sought ways tominimize the amount of parasitic reactions taking place in a givenelectrochemical cell, such as within a flow battery. Surprisingly, theinventors discovered a simple modification to conventionalelectrochemical cell designs that can mitigate the occurrence ofparasitic reactions without greatly altering manufacturing processesused for constructing the electrochemical cells. Mass transportresistance can also be lessened in the modified electrochemical celldesigns. More specifically, the inventors discovered that by fabricatingan electrochemical cell with an electrode having one face that is morehydrophilic than is the other, the incidence of parasitic reactions canbe decreased. The more hydrophilic face of the electrode is disposedadjacent to the separator in the electrochemical cells, therebypromoting preferential convective flow of an aqueous electrolytesolution in a hydrophilic region proximate the separator. Parasiticreactions are generally less prevalent near the separator, and bypreferentially circulating the electrolyte solution near the separator,the overall incidence of parasitic reactions can be decreased byincreasing the prevalence of productive reactions. Correspondingly, bydecreasing the extent of electrolyte circulation at locales more removedfrom the separator, the incidence of parasitic reactions can also belessened. Advantageously, facile manufacturing techniques are availablefor processing a conductive material for an electrode so that gradienthydrophilicity results. Further details in this regard followhereinbelow.

The electrochemical cells described herein are particularly advantageousfor mitigating parasitic reactions, since conventional cell designsoffer no appreciable mechanism for minimizing the incidence of parasiticreactions other than by changing the cell's operating conditions suchthat an overpotential for parasitic reactions is not exceeded and/or bychanging the cell's chemical composition altogether. By changing anelectrochemical cell in this manner, however, productive reactions of anactive material may not occur at all, or they may not occur withsufficient rapidity. Thus, changing the operating conditions of a flowbattery or other electrochemical system to mitigate parasitic reactionscan be an untenable approach in many circumstances. In contrast, theelectrochemical cells of the present disclosure address the incidence ofparasitic reactions without requiring a significant change in celldesign, composition, and/or operating conditions. Before discussingfurther specifics of the electrochemical cells of the presentdisclosure, illustrative flow battery configurations and their operatingcharacteristics will first be described in greater detail hereinafter.

Unlike typical battery technologies (e.g., Li-ion, Ni-metal hydride,lead-acid, and the like), where active materials and other componentsare housed in a single assembly, flow batteries transport (e.g., viapumping) redox-active energy storage materials from storage tanksthrough an electrochemical stack containing one or more electrochemicalcells. This design feature decouples the electrical energy storagesystem power from the energy storage capacity, thereby allowing forconsiderable design flexibility and cost optimization. FIG. 1 shows aschematic of an illustrative flow battery containing a singleelectrochemical cell. Although FIG. 1 shows a flow battery containing asingle electrochemical cell, approaches for combining multipleelectrochemical cells together are known and are discussed hereinbelow.

As shown in FIG. 1, flow battery system 1 includes an electrochemicalcell that features separator 20 between electrodes 10 and 10′ of theelectrochemical cell. As used herein, the terms “separator” and“membrane” refer to an ionically conductive and electrically insulatingmaterial disposed between the positive and negative electrodes of anelectrochemical cell. The two terms are used synonymously herein.Electrodes 10 and 10′ are formed from a suitably conductive material,such as a metal, carbon, graphite, and the like, and the materials fortwo can be the same or different. Although FIG. 1 has shown electrodes10 and 10′ as being spaced apart from separator 20, electrodes 10 and10′ can also be abutted with separator 20 in more particular embodiments(see FIG. 2 below). The material(s) forming electrodes 10 and 10′ can beporous, such that they have a high surface area for contacting theelectrolyte solutions containing first active material 30 and secondactive material 40, which are capable of being cycled between anoxidized state and a reduced state. For example, one or both ofelectrodes 10 and 10′ can be formed from a porous carbon cloth or acarbon foam in some embodiments.

Pump 60 affects transport of first active material 30 from tank 50 tothe electrochemical cell. The flow battery also suitably includes secondtank 50′ that contains second active material 40. Second active material40 can he the same material as first active material 30, or it can bedifferent. Second pump 60′ can affect transport of second activematerial 40 to the electrochemical cell. Pumps can also be used toaffect transport of active materials 30 and 40 from the electrochemicalcell back to tanks 50 and 50′ (not shown in FIG. 1). Other methods ofaffecting fluid transport, such as siphons, for example, can alsosuitably transport first and second active materials 30 and 40 into andout of the electrochemical cell. Also shown in FIG. 1 is power source orload 70, which completes the circuit of the electrochemical cell andallows a user to collect or store electricity during its operation.

It should be understood that FIG. 1 depicts a specific, non-limitingconfiguration of a particular flow battery. Accordingly, flow batteriesconsistent with the spirit of the present disclosure can differ invarious aspects relative to the configuration of FIG. 1. As one example,a flow battery system can include one or more active materials that aresolids, gases, and/or gases dissolved in liquids. Active materials canbe stored in a tank, in a vessel open to the atmosphere, or simplyvented to the atmosphere.

As indicated above, multiple electrochemical cells can also be combinedwith one another in an electrochemical stack in order to increase therate that energy can be stored and released during operation. The amountof energy released is determined by the overall amount of activematerial that is present. An electrochemical stack utilizes bipolarplates between adjacent electrochemical cells to establish electricalcommunication but not fluid communication between the two cells. Thus,bipolar plates contain the electrolyte solutions within the individualelectrochemical cells. Bipolar plates are generally fabricated fromelectrically conductive materials that are fluidically non-conductive onthe whole. Suitable materials can include carbon, graphite, metal, or acombination thereof. Bipolar plates can also be fabricated fromnon-conducting polymers having a conductive material dispersed therein,such as carbon particles or fibers, metal particles or fibers, graphene,and/or carbon nanotubes. Although bipolar plates can be fabricated fromthe same types of conductive materials as can the electrodes of anelectrochemical cell, they can lack the continuous porosity permittingan electrolyte solution to flow completely through the latter. It shouldbe recognized that bipolar plates are not necessarily entirelynon-porous entities, however. Bipolar plates can have innate or designedflow channels that provide a greater surface area for allowing anelectrolyte solution to contact the bipolar plate, but the porousfeatures terminate at a location before the electrolyte solution canenter an adjacent electrochemical cell. Suitable flow channelconfigurations can include, for example, interdigitated flow channels(see FIG. 3). In some embodiments, the flow channels can be used topromote delivery of an electrolyte solution to an electrode within theelectrochemical cell. Delivery of an electrolyte solution to anelectrode via a bipolar plate is discussed in more detail hereinbelow.

Materials for forming the bipolar plates used in the various embodimentsof the present disclosure are not considered to be particularly limited,other than having sufficient electrical conductivity to establishelectrical communication between adjacent cells in an electrochemicalstack. Cost and ease of machining to produce a desired shape or flowchannel geometry can also be considerations for choosing a particularconductive material over another. In more specific embodiments, thebipolar plate can be formed from a graphite block. Alternative materialsfor fabricating a bipolar plate can include, for example, conductivecomposites containing a binder and a conductive filler. In illustrativeembodiments, the binder can be a polymer such as polyvinyl acetate,polytetrafluoroethylene or the like, and the conductive filler can be acarbon-based material such as carbon powder, carbon nanotubes, graphene,or any combination thereof. The bipolar plate can also have a pluralityof flow channels defined therein, such as a plurality of interdigitatedflow channels. The flow channels can be configured to deliver a firstelectrolyte solution to the first electrode and a second electrolytesolution to the second electrode.

FIG. 2 shows a schematic of an illustrative electrochemical cellconfiguration containing a bipolar plate abutting each electrode. Whereappropriate, common reference characters will be used to describeelements shown in a preceding FIGURE. Referring to FIG. 2, negativehalf-cell 80 and positive half-cell 80′ are disposed on opposing sidesof separator 20. Negative half-cell 80 contains electrode 10 (i.e., theanode) abutted with separator 20 at interface 12, and bipolar plate 90is, in turn, abutted against the opposing face of electrode 10 atinterface 14. Positive half-cell 80′ similarly contains electrode 10′(i.e., the cathode) abutted with the opposing face of separator 20 atinterface 12′, and bipolar plate 90′ is, in turn, abutted against theopposing face of electrode 10′ at interface 14′. Flow channels 82 extendpartially within the interior of bipolar plates 90 and 90′ and increasethe degree of contact with the electrolyte solution. In someembodiments, flow channels 82 can be in an interdigitated configurationas shown in FIG. 3 below. Other configurations for flow channelsinclude, for example, regular or irregular spacing, randomdirectionality, tortuous interconnected pathways, random distributionsand/or gradient distributions. In the interest of clarity, the fluidflow details shown in FIG. 1 are not presented in FIG. 2. However, itcan be readily appreciated how the electrochemical cell configuration ofFIG. 2 would be incorporated in FIG. 1, or how a plurality ofelectrochemical cells would be incorporated an electrochemical stack andconnected to a fluid distribution manifold to deliver an electrolytesolution. For example, a fluid distribution manifold can be connected toan inlet on bipolar plates 90 and 90′ to supply an electrolyte solutionto electrodes 10 and 10′, as shown hereinafter. For purposes ofdiscussion herein, the electrochemical cell configuration of FIG. 2 willbe considered representative of that present in a conventional flowbattery.

FIG. 3 shows an illustrative schematic of a bipolar plate containinginterdigitated flow channels. As shown in FIG. 3, bipolar plate 90includes inlet channel 91 and outlet channel 92, and flow channels 82are interdigitated with one another in between. Thus, a fluiddistribution manifold (not shown) can be connected to inlet channel 91to supply an electrolyte solution to alternating flow channels 82. Afterinteracting with electrode 10, the electrolyte solution can migrate viaconvective flow to flow channels 82 beside those that are initiallyfilled with electrolyte solution, and the electrolyte solution thenexits bipolar plate 90 via outlet channel 92.

As indicated above, the vast majority of productive reactions inconventional electrochemical cells occur at or in close proximity to theinterface between the electrodes and the separator (i.e., at interfaces12 and 12′ in FIG. 2). Parasitic reactions, in contrast, are moreprevalent at locations that are more distant from the separator. Bypreferentially supplying an electrolyte solution to a region whereproductive reactions are more prevalent, the overall ratio of productivereactions to parasitic reactions can be increased. In the case of thepresent disclosure, the inventors discovered that by including anelectrode having a hydrophilic face disposed adjacent to or abutting theseparator, increased convective flow of the corresponding electrolytesolution near the separator can be realized when the opposing electrodeface is less hydrophilic (i.e., more hydrophobic), such that gradienthydrophilicity exists in the electrode. The resulting decrease in masstransport resistance within the electrochemical cell can be substantial.Either or both of the electrodes in a flow battery can be modified toinclude gradient hydrophilicity, depending upon the half-cell(s) inwhich one wishes to mitigate the incidence of parasitic reactions.Without being bound by theory or mechanism, it is believed that theenhanced circulation of aqueous electrolyte solutions near the separatorarises due to “like-complements-like” matching between the hydrophilicelectrode face and the hydrophilic aqueous electrolyte solution.

In alternative embodiments of the present disclosure, wherein organicelectrolyte solutions containing non-polar solvents are used, it can bemore desirable to reverse the polarity of the first face and the secondface of the electrodes. Specifically, in the case of a non-polar solventbeing present, the first face proximate to the separator can be mademore hydrophobic, and the second face proximate the bipolar plate can bemade more hydrophilic. Modifying the electrode in this manner can directconvective flow of the non-polar solvent to a non-polar region proximatethe separator to promote decreased parasitic reactions and decreasedmass transfer resistance similar to that discussed above.

As indicated above, the electrode modifications of the presentdisclosure advantageously do not represent a dramatic change inconventional electrochemical cell architectures. Thus, electrodes havinggradient hydrophilicity can be utilized as essentially a drop-inreplacement in conventional electrochemical cell manufacturingprocesses. Further, a variety of manufacturing techniques can besuitable for modifying the conductive material of an electrode to renderone face more hydrophilic than the other. Plasma spray functionalization(e.g., oxygen plasma treatment) of one face of a planar conductivematerial, such as a carbon cloth, can represent a particularly faciletechnique for increasing hydrophilicity. Further advantageously, aconductive material can be modified to introduce gradient hydrophilicityeither prior to manufacturing processes for constructing anelectrochemical cell, or in an integrated manner within anelectrochemical cell manufacturing process. Thus, considerableoperational flexibility can be realized when fabricating anelectrochemical cell according to the disclosure herein.

Accordingly, in various embodiments, flow batteries of the presentdisclosure can include a first half-cell containing a first electrodewith a first face and a second face that are directionally opposite oneanother, a second half-cell containing a second electrode with a firstface and a second face that are directionally opposite one another, anda separator disposed between the first half-cell and thesecond-half-cell. The first face of both the first electrode and thesecond electrode are disposed adjacent to the separator. The first faceof at least one of the first electrode and the second electrode is morehydrophilic than is the second face. In some embodiments, the first faceof both the first and second electrodes can be more hydrophilic than isthe second face.

In further embodiments, the flow batteries can include a first bipolarplate contacting the second face of the first electrode and a secondbipolar plate contacting the second face of the second electrode. Insome more specific embodiments, the first bipolar plate and the secondbipolar plate can each contain a plurality of flow channels, in whichthe plurality of flow channels are configured to deliver a firstelectrolyte solution to the first electrode and a second electrolytesolution to the second electrode. In more particular embodiments, thefirst electrolyte solution and/or the second electrolyte solution can bean aqueous electrolyte solution.

Accordingly, flow batteries of the present disclosure have ahydrophilicity gradient (also referred to herein as gradienthydrophilicity) in at least one of the first electrode and the secondelectrode, which extends outwardly from the separator. That is, at leastone of the first electrode and the second electrode have ahydrophilicity gradient in which the hydrophilicity decreases outwardlyfrom the separator (i.e., between the separator and the bipolar plate).The hydrophilicity gradient can be a continuous gradient in someembodiments, or a stepped gradient in other embodiments. Regardless ofthe gradient type that is present, the higher electrode hydrophilicityin proximity to the separator can allow a decreased incidence ofparasitic reactions to be realized, as described hereinabove.

In some embodiments, the hydrophilicity gradient extending between theseparator and the first and/or second bipolar plate can be a continuousgradient. In a continuous gradient, once the hydrophilicity startsdecreasing, it continually decreases (i.e., without the rate of changebecoming zero) between the separator and the corresponding bipolarplate. The rate of change can vary in a continuous hydrophilicitygradient, however, without becoming zero. The hydrophilicity of theelectrode(s) may, in some embodiments, be constant in a region proximatethe separator before the hydrophilicity begins continually decreasing ata location spaced apart from the separator.

In other embodiments, the hydrophilicity gradient can be a steppedgradient. In a stepped gradient, the hydrophilicity can decrease oversome distance from the separator, and then the rate of change can becomezero at some distance from the separator before possibly decreasing yetagain. Again, the hydrophilicity may, in some embodiments, be constantin a region proximate the separator before the density begins decreasingat a location spaced apart from the separator. Stepped gradients can beintroduced through various facile manufacturing processes, as discussedhereinbelow.

A number techniques and scales are available to quantify thehydrophilicity of various substances. The specific technique and scalecan be specific to a particular application. For example, contact angleswith water can be applicable when classifying a surface, whereas log Pvalues are more applicable when classifying the hydrophobicity of apharmaceutical. In the various embodiments of the present disclosure,the absolute hydrophilicity or hydrophobicity values of the first faceand the second face are not considered to be particularly critical.Instead, the present disclosure simply provides that the first face ismore hydrophilic than is the second face, and one having ordinary skillin the art will be able to ascertain readily whether certain surfacemodifications to the first face and/or the second face render one facemore hydrophilic than the other. For example, an electrode faceincorporating heteroatoms such as oxygen and/or nitrogen will be readilyunderstood by one having ordinary skill in the art to be morehydrophilic than is an unmodified electrode face. Oxygen and nitrogenfunctionalities that are capable of donating and/or receiving hydrogenbonds can be particularly facile for increasing the hydrophilicity of anelectrode face upon which they are present.

Particular hydrophobicity scales and techniques applicable for assayingthe electrode faces of the present disclosure can include, for example,water absorption rates, contact angles, and cyclic voltammetry values.In the case of contact angles with water, for example, an acute contactangle represents a hydrophilic surface, and an obtuse contact anglerepresents a hydrophobic surface. In a relative sense, differing contactangles for one surface compared to another represent differinghydrophilicity/hydrophobicity values. For example, in the variousembodiments of the present disclosure, an electrode face proximate tothe separator would have a smaller contact angle with water than wouldthe electrode face proximate a bipolar plate.

The manner in which the first face of the electrode(s) is rendered morehydrophilic than is the second face is not considered to be particularlylimited. In some embodiments, one face of a conductive material canundergo a selective hydrophilic modification to render the first face ofthe electrode more hydrophilic. In other embodiments, one face of aconductive material can undergo a selective hydrophobic modification torender the second face of the electrode more hydrophobic, thereby makingthe first face of the electrode more hydrophilic in the process. Thatis, by hydrophobically modifying the second face of the electrode (i.e.,the face that is spaced apart from the separator), the hydrophilicity ofthe first face relative to the second face can be increased. In someembodiments, a conductive additive deposited upon the second face of theelectrode can increase the relative hydrophilicity of the first face inthe foregoing manner.

Depending on the particular half-cell in which one wants to mitigate theoccurrence of parasitic reactions, one or both of the electrodes can bemodified to have a first face that is more hydrophilic than is a secondface, thereby generating gradient hydrophilicity in the electrode(s). Insome embodiments, the electrodes in both the first half-cell and thesecond half-cell can have hydrophilicity values that decrease outwardlyfrom the separator. In other embodiments, the first electrode in thefirst half-cell can have such decreasing hydrophilicity values, and thesecond electrode in the second half-cell can have substantially equalhydrophilicity values at both faces of the electrode, such as in thecase of unmodified electrode surfaces. The first electrode can be eitherthe anode or the cathode. In the drawings that follow, both electrodeswill be depicted as having a hydrophilicity gradient, but it is to berecognized that the electrodes can be modified individually as well.Modification of the electrode in the negative half-cell of a flowbattery or similar electrochemical device can be especially desirable,since the pH changes accompanying parasitic generation of hydrogen canbe particularly detrimental.

FIG. 4 shows a schematic of an illustrative electrochemical cellconfiguration in which the relative hydrophilicity of the electrodes hasbeen increased proximate the separator. As shown in FIG. 4, electrode 10includes hydrophilic surface 101 in proximity to separator 20.Hydrophobic remainder 100 of electrode 10 presents a more hydrophobicsurface at interface 14 in proximity to bipolar plate 90. Similarly,hydrophilic surface 101′ and hydrophobic remainder 100′ are locatedwithin electrode 10′. Other than having the foregoing hydrophilicitygradient within electrodes 10 and 10′, the electrochemical cellconfiguration of FIG. 4 is identical to that of FIG. 2 and may be betterunderstood by reference thereto. As indicated above, hydrophilicsurfaces 101 and 101′ provide a more effective electrode surface areafor convective flow of an aqueous electrolyte solution adjacent toseparator 20.

FIGS. 5A and 5B show illustrative schematics demonstrating differencesin fluid flow dynamics in the absence and in the presence of anelectrode having a hydrophilicity gradient. In the interest ofconciseness, FIGS. 5A and 5B show only show a single half-cell, but itis to be recognized that the concepts of these FIGURES can be easilyextended to a full electrochemical cell in a flow battery.

FIG. 5A shows a half-cell of a flow battery in which electrode 10 issubstantially unmodified and does not have a hydrophilicity gradient. Asshown in FIG. 5A, an electrolyte solution is supplied to electrode 10through bipolar plate 90, which contains interdigitated flow channels.Specifically, the electrolyte solution enters through flow channels 82a, penetrates some distance into electrode 10, and then exits throughflow channels 82 b. Since electrode 10 does not vary in hydrophilicityin FIG. 5A, the penetration depth of the electrolyte solution can berelatively small before the electrolyte solution exits through flowchannels 82 b. As a result, only a small fraction of the electrolytesolution reaches separator 20 in order for productive reactions to takeplace.

In contrast, FIG. 5B shows a half-cell of a flow battery in whichelectrode 10 does have a hydrophilicity gradient, specificallyhydrophilic surface 101 adjacent to separator 20 and hydrophobicremainder 100 adjacent to bipolar plate 90. That is, hydrophobicremainder 100 is sandwiched between bipolar plate 90 and hydrophilicsurface 101, which abuts separator 20. As in FIG. 5A, electrolytesolution enters through flow channels 82 a, penetrates some distanceinto electrode 10 and then exits through flow channels 82 b. In thiscase, however, the penetration depth is much deeper and closer toseparator 20 due to the increased complementarity of an aqueouselectrolyte solution to hydrophilic surface 101.

FIG. 6 shows a comparative plot of calculated cell resistance for ahomogenous electrode and an engineered gradient electrode as a functionof state of charge. As shown in FIG. 6, the gradient electrode canproduce a lower cell resistance at all states of charge.

In more particular embodiments, at least one of the first electrode andthe second electrode can be a carbon electrode. For example, in someembodiments, the first electrode and/or the second electrode can beformed from a carbon cloth. Numerous examples of carbon cloths suitablefor forming a carbon electrode will be familiar to one having ordinaryskill in the art. One face of a carbon cloth can be preferentially madehydrophilic or hydrophobic to render the first surface of the electrodemore hydrophilic in accordance with the present disclosure.

As indicated above, various modes are contemplated by the presentdisclosure for rendering the first face of one or more of the electrodesmore hydrophilic than the second face. More specifically, the first facecan be made more hydrophilic than is a remaining portion of theelectrode, or the second face can be made more hydrophobic than is aremaining portion of the electrode. Increasing the hydrophilicity of thefirst face of the electrode(s) can be a particularly desirable solution,since modifying the first face of the electrode in this manner canimprove complementarity with an aqueous electrolyte solution.

Techniques for inducing gradient hydrophilicity in an electrode orconductive material are discussed in greater detail hereinbelow. At thisjuncture, modifications resulting in gradient hydrophilicity within anelectrode will be discussed in terms of structure and composition,rather than in terms of fabrication methodology.

In some embodiments, at least one of the first electrode and the secondelectrode can include a conductive additive deposited upon the secondface (i.e., opposite the separator) to a greater extent than upon thefirst face. Masking and air oxidation of the first and/or second facecan be used in conjunction with such embodiments. As used herein, thephrase “to a greater extent” refers to a modification operation thatoccurs selectively at one location over another. Selective modificationdoes not necessarily exclude some degree of modification on the firstsurface (i.e., adjacent to the separator) in the case of a conductiveadditive, although that can indeed be the case. Suitable methodsdepositing the conductive additive are discussed hereinbelow.

In more particular embodiments, the conductive additive deposited uponthe second surface can include materials such as, for example, amorphouscarbon, graphite, carbon nanotubes, graphene, or any combinationthereof. Some metals can also be suitable for use in this regard.Particularly in the case of non-carbon electrodes, these carbonnanomaterials can be particularly efficacious in increasinghydrophobicity of the second surface. Even when the electrode is acarbon electrode, some degree of increased hydrophobicity can still berealized by depositing these carbon nanomaterials upon the second face.In some embodiments, a loading of the conductive additive upon thesecond face of an electrode can range between about 0.1% to about 50% byweight of the electrode, or about 0.5% to about 25% by weight, or about1% to about 10% by weight.

Suitable carbon nanotubes for incorporation upon the second face of anelectrode can include single-wall carbon nanotubes, double-wall carbonnanotubes, multi-wall carbon nanotubes, or any combination thereof. Thecarbon nanotubes can be metallic, semimetallic, or semiconductingdepending on their chirality. An established system of nomenclature fordesignating a carbon nanotube's chirality is recognized in the art andis distinguished by a double index (n, m), where n and m are integersthat describe the cut and wrapping of hexagonal graphite when formedinto a tubular structure. In addition to chirality, a carbon nanotube'sdiameter also influences its electrical and thermal conductivity values.Multi-wall carbon nanotubes typically have more complex electrical andthermal conductivity profiles than do single-wall carbon nanotubes dueto interwall reactions that can occur between the individual nanotubelayers. By contrast, there is no change in the electrical and thermalconductivity profiles across different portions of a single-wall carbonnanotube. Accordingly, in more particular embodiments of the presentdisclosure, the conductive additive can include a plurality ofsingle-wall carbon nanotubes due to their favorable conductivity values.

In other various embodiments, a carbon electrode can have gradienthydrophilicity introduced therein without depositing a conductiveadditive upon the second face. More specifically, in some embodiments ofthe present disclosure, at least one of the first electrode and thesecond electrode can be a carbon electrode in which the first face orthe second face of the carbon electrode is functionalized. In someembodiments, a carbon electrode can be functionalized upon a first facewith a plurality of hydrophilic molecules to a greater extent than uponthe second face, thereby rendering the first face more hydrophilic. Inother embodiments, a carbon electrode can be functionalized on a secondface with a plurality of hydrophobic molecules to a greater extent thanupon the first face, thereby rendering the first face more hydrophilicon a relative basis. It is again to be emphasized that the specificidentity of the hydrophilic molecules or hydrophobic molecules is notconsidered to be particularly relevant to the embodiments of the presentdisclosure. Instead, it is to be recognized that the classification ofvarious molecules as being hydrophilic or hydrophobic is dependent uponthe native characteristics of the surface and the manner in which thehydrophilic/hydrophobic molecules react with the carbon surface. Forexample, an otherwise hydrophilic molecule that reacts with a carbonsurface through loss of a heteroatom functional group to leave onlycarbon atoms upon the electrode surface would be classified as ahydrophobic molecule according to the various embodiments of the presentdisclosure. Thus, the classification of particular molecules as beinghydrophilic or hydrophobic can change depending upon the nature of theelectrode surface undergoing modification with the molecules.

In one non-limiting example, surface oxides upon a carbon electrode canbe exploited to alter the relative hydrophilicity of one face of acarbon electrode. As used herein, the term “surface oxide” refers tooxygen-containing functional groups that are natively present upon thesurface of a carbon material. Surface oxides can include, but are notlimited to, hydroxyl groups, carbonyl groups, epoxide groups, carboxylicacid groups or the like. Any of these functional groups can be reactedupon one face of the carbon electrode to alter the relativehydrophilicity.

FIGS. 7 and 8 show illustrative schematics demonstrating how surfaceoxides in a carbon material can be manipulated to alter thehydrophilicity of one electrode face to a greater extent than the other.In the interest of simplicity, FIGS. 7 and 8 show modification of thecarbon material (e.g., a carbon cloth) before its incorporation in anelectrochemical cell. As shown in FIG. 7, carbon material 200 includessurface oxides 210 upon opposing first face 220 and second face 230.Following selective reaction of hydrophilic molecules 240 with at leasta portion of surface oxides 210 upon first face 220, first face 220 isrendered more hydrophilic than is the rest of carbon material 200, whichremains unmodified (i.e., second face 230). Likewise, as shown in FIG.8, following selective reaction of hydrophobic molecules 250 with atleast a portion of surface oxides 210 upon second face 230, second face230 is rendered more hydrophobic compared to the remainder of carbonmaterial 200, which remains unmodified (i.e., first face 220). As aresult, first face 220 becomes more hydrophilic on a relative basis inFIG. 8. Although FIGS. 7 and 8 have depicted only surface hydroxylgroups as being present, it is to be understood that other types ofsurface oxides can also be present, as discussed above, and can undergodifferent types of functionalization. Moreover, hydrophilic molecules240 and hydrophobic molecules 250 shown in FIGS. 7 and 8 are exemplaryin nature and should be considered non-limiting. As discussed above, onehaving ordinary skill in the art can envision a variety of hydrophilicor hydrophobic molecules appropriate for reaction with a particular typeof electrode surface in order to render one face of the electrode morehydrophilic than the other.

In alternative embodiments, one face of a carbon electrode can beselectively functionalized to alter the hydrophobicity or hydrophilicitywithout making use of the surface oxides to promote functionalization.For example, in some embodiments, diazonium salt can be used tofunctionalize the basal plane of a carbon surface directly.

In still other embodiments, physical treatments can be used to alter thehydrophobicity or hydrophilicity of one face of a carbon electrode orother conductive material to a greater extent than another face (i.e.,without undergoing a reaction with a hydrophobic or hydrophilic speciesthat becomes incorporated upon the electrode face). In some embodiments,plasma functionalization can be used to functionalize the first face ofthe electrode to a greater extent than is the second face. In someembodiments, plasma functionalization can take place with exclusion ofplasma-introduced hydrophilic functional groups upon the second face.Specifically, in the case of a carbon electrode, plasmafunctionalization of the first face can introduce additional oxygenatedfunctionalities to the first face but not substantially to the secondface, thereby rendering the first face more hydrophilic. The oxygenatedfunctionalities introduced via a plasma can include similarfunctionalities to those present in the surface oxides of a carbonelectrode, although different functionalities can also be introducedthrough plasma functionalization, and the overall distribution ofoxygenated functionality types can differ compared to the nativedistribution of types in the surface oxides. Plasma functionalizationtechniques can be particularly desirable in the context of the presentdisclosure due to the efficacy with which a plasma spray can be directedonto one face of a conductive material without substantially contactingthe plasma spray with the other face.

FIG. 9 shows an illustrative schematic demonstrating how plasmafunctionalization can be used to increase the hydrophilicity of oneelectrode face to a greater extent than the other. In the interest ofclarity, surface oxides are again shown in FIG. 9 to demonstrate thatplasma functionalization selectively increases the amount of oxygenatedfunctionalities upon one face of the electrode compared to the other. Itis to be recognized, however, that a carbon electrode substantiallydevoid of surface oxides can be plasma functionalized in a similarmanner. As shown in FIG. 9, carbon material 200 includes an initialdistribution of surface oxides upon first face 220 and second face 230.Plasma sprayer 260 is configured to direct plasma spray 270 onto firstface 220 without the plasma spray substantially interacting with secondface 230. Thus, as shown in FIG. 9, a different distribution ofoxygenated functionalities is obtained on first face 220 followingplasma functionalization, thereby rendering first face 220 morehydrophilic than is the unmodified second face 230. It is again to beemphasized that the initial surface oxides may or may not be present,and the amount, type and distribution of oxygenated functionalitiesintroduced through plasma treatment can vary to some degree. Forexample, the plasma energy and time of exposure of a carbon material tothe plasma spray can influence the distribution and type of oxygenatedfunctionalities that are obtained upon first face 220. Hence, FIG. 9should be understood to be representative of certain non-limitingembodiments of the present disclosure.

In alternative embodiments, an electrode can be fabricated with a firstface that is more hydrophilic than is a second face by combining twoconductive materials having differing hydrophilicity characteristics.Specifically, in some embodiments, two conductive materials havingdifferent hydrophilicity characteristics can be stacked or layered uponeach other to form an electrode in which the first face is morehydrophilic than is the second face. Generally, the electrode contains afirst layer of a first conductive material and a second layer of asecond conductive material, although additional layers can also bepresent in some embodiments.

In more specific embodiments, carbon cloths having differinghydrophilicity characteristics can be layered together to form a carbonelectrode having faces that differ in hydrophilicity. More particularly,in some embodiments, at least one of the first electrode and the secondelectrode can include a first carbon cloth and a second carbon cloththat are layered together such that the first carbon cloth is adjacentto the separator, and the first carbon cloth is more hydrophilic than isthe second carbon cloth. In such embodiments, the second carbon clothcan abut the bipolar plate. The hydrophilic or hydrophobiccharacteristics of each carbon cloth can be modified, if needed, inmanners similar to those described above.

In some or other embodiments, the electrochemical cells andelectrochemical stacks disclosed herein can be incorporated in flowbatteries or similar electrochemical systems. Exemplary flow batteryconfigurations are discussed in more detail hereinabove. Otherelectrochemical systems in which the electrochemical cells andelectrochemical stacks of the present disclosure can be applicableinclude, for example, electrolyzers and fuel cell systems.

Accordingly, methods for decreasing the incidence of parasitic reactionswithin a flow battery are implicitly described herein. In variousembodiments, such methods can include: providing a flow battery havingan electrochemical cell including a first half-cell containing a firstelectrode with a first face and a second face that are directionallyopposite one another, a second half-cell containing a second electrodewith a first face and a second face that are directionally opposite oneanother, and a separator disposed between the first half-cell and thesecond half-cell, and circulating a first electrolyte solution throughthe first half-cell and a second electrolyte solution through the secondhalf-cell. The first face of both the first and second electrodes isdisposed adjacent to the separator, and the first face of at least oneof the first electrode and the second electrode is more hydrophilic thanis the first face. Convective flow of at least one of the firstelectrolyte solution and the second electrolyte solution occurspreferentially in a hydrophilic region of the first electrode or thesecond electrode proximate the separator. In more specific embodiments,the first electrolyte solution and the second electrolyte solution canbe supplied through a plurality of flow channels within a first bipolarplate contacting the first electrode and a second bipolar platecontacting the second electrode, respectively (see FIGS. 3 and 5B).

In further embodiments, a plurality of the electrochemical cells can beconnected in series with one another in an electrochemical stack. Thebipolar plates from adjacent electrochemical cells can abut one another,or a bipolar plate can be shared between adjacent electrochemical cells.

In related embodiments, methods for fabricating an electrochemical cellhaving at least one electrode with gradient hydrophilicity are alsodescribed herein. The methods can include: providing a conductivematerial with a first face and a second face that are directionallyopposite one another, in which the first face is more hydrophilic thanis the second face, and forming an electrochemical cell therefrom. Theelectrochemical cell includes a first half-cell containing a firstelectrode, a second half-cell containing a second electrode, and aseparator disposed between the first half-cell and the second half-cell.At least one of the first electrode and the second electrode includesthe conductive material, and the first face of the conductive materialis disposed adjacent to the separator.

In further embodiments, a first bipolar plate contacts the firstelectrode opposite the separator and a second bipolar plate contacts thesecond electrode opposite the separator. Accordingly, in still furtherembodiments, the methods can include connecting a plurality ofelectrochemical cells in series with one another in an electrochemicalstack. In some or other embodiments, the electrochemical cell can bepresent in a flow battery.

In some embodiments, the methods can include functionalizing the firstface of the conductive material to a greater extent than the secondface, thereby rendering the first face more hydrophilic. As discussedabove, plasma functionalization techniques can be particularly suitablefor this purpose, particularly for functionalizing carbon electrodes,due to the specificity with which they can be directed onto one face ofthe conductive material over another. Other types of selective facialfunctionalization can also be accomplished with care. For example, insome embodiments, selective facial masking and de-masking can be used toaccomplish functionalization of one face over another. Suitable maskingtechniques will be familiar to one having ordinary skill in the art.

In alternative embodiments, the methods of the present disclosure caninclude functionalizing the second face of the conductive material to agreater extent than the first face with a plurality of hydrophobicmolecules, thereby rendering the first face more hydrophilic. Techniquesfor functionalizing the first face can be readily adapted and/orcombined by one having ordinary skill in the art for purposes offunctionalizing the second face.

As discussed above, a conductive additive deposited upon the second faceof the conductive material can also increase hydrophobicity of thesecond face, thereby increasing the relative hydrophilicity of the firstface. When depositing a conductive additive upon the second face of oneor more of the electrodes, chemical vapor deposition (CVD) can be aparticularly suitable deposition technique. CVD techniques can beparticularly useful for depositing conductive additives such as, forexample, amorphous carbon, graphite, carbon nanotubes, and graphene.Suitable CVD techniques for these materials will be familiar to onehaving ordinary skill in the art and need not be described in furtherdetail herein.

Other suitable deposition techniques for applying a conductive additiveto a conductive material can include applying a solvent dispersion ofthe conductive additive onto the second face of the conductive material.For example, in some embodiments, a solvent dispersion of the conductiveadditive can be applied to the conductive material by spraying,painting, and/or dip coating techniques. Again, details regarding suchdeposition techniques will be familiar to one having ordinary skill inthe art.

In still other alternative embodiments, the conductive material need notnecessarily undergo modification at all in order to introduce gradienthydrophilicity into an electrode. Instead, two or more conductivematerials having different hydrophilicity characteristics can be layeredupon one another to produce a hydrophilicity gradient. For example, inthe case of a carbon cloth, a first carbon cloth and a second carboncloth can be layered together, such that the first carbon cloth isadjacent to the separator, and the first carbon cloth is morehydrophilic than is the second carbon cloth. Accordingly, the firstcarbon cloth is adjacent to or contacts the separator, and the secondcarbon cloth is spaced apart from the separator, such that it lies inproximity to the bipolar plate, if present.

As indicated above, the various embodiments of the present disclosurecan desirably decrease the incidence of parasitic reactions that occurwithin an electrochemical cell. More particularly, the embodiments ofthe present disclosure can provide a ratio of productive reactions toparasitic reactions at least exceeding 5 when the flow battery is inoperation. Higher ratios of productive reactions to parasitic reactionscan also be realized. In some embodiments, a ratio of productivereactions to parasitic reactions can be about 10 or above, or about 20or above, or about 30 or above, or about 40 or above, or about 50 orabove, or about 100 or above, or about 200 or above, or about 500 orabove, or about 1000 or above. A suitable ratio of productive reactionsto parasitic reactions can vary from application to application.Accordingly, the design of a given electrochemical cell can incorporateany number of the features described hereinabove to achieve a givenratio of productive reactions to parasitic reactions. A suitable ratiocan be determined for a given application based upon optimizing currentefficiency versus cell resistance.

In some embodiments, flow batteries of the present disclosure caninclude an active material in one or more electrolyte solutions that isa coordination complex. As used herein, the terms “coordination complex”and “coordination compound” refer to any compound having a metal boundto one or more ligands through a covalent bond. Due to their variableoxidation states, transition metals can be highly desirable for usewithin the active materials of a flow battery. Cycling between theaccessible oxidation states can result in the conversion of chemicalenergy into electrical energy. Lanthanide metals can be used similarlyin this regard in alternative embodiments. Particularly desirabletransition metals for inclusion in a flow battery include, for example,Al, Cr, Ti and Fe. For purposes of the present disclosure, Al is to beconsidered a transition metal. In some embodiments, coordinationcomplexes within a flow battery can include at least one catecholate orsubstituted catecholate ligand. Sulfonated or hydroxylated catecholateligands can be particularly desirable ligands due to their ability topromote solubility of coordination complexes in which they are present.

Other ligands that can be present in coordination complexes, alone or incombination with one or more catecholate or substituted catecholateligands, include, for example, ascorbate, citrate, glycolate, a polyol,gluconate, hydroxyalkanoate, acetate, formate, benzoate, malate,maleate, phthalate, sarcosinate, salicylate, oxalate, urea, polyamine,aminophenolate, acetylacetonate, and lactate. Where chemically feasible,it is to be recognized that such ligands can be optionally substitutedwith at least one group selected from among C₁₋₆ alkoxy, C₁₋₆ alkyl,C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5- or 6-membered aryl or heteroaryl groups,a boronic acid or a derivative thereof, a carboxylic acid or aderivative thereof, cyano, halide, hydroxyl, nitro, sulfonate, asulfonic acid or a derivative thereof, a phosphonate, a phosphonic acidor a derivative thereof, or a glycol, such as polyethylene glycol.Alkanoate includes any of the alpha, beta, and gamma forms of theseligands. Polyamines include, but are not limited to, ethylenediamine,ethylenediamine tetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA).

Other examples of ligands can be present include monodentate, bidentate,and/or tridentate ligands. Examples of monodentate ligands that can bepresent in a coordination complex include, for example, carbonyl orcarbon monoxide, nitride, oxo, hydroxo, water, sulfide, thiols,pyridine, pyrazine, and the like. Examples of bidentate ligands that canbe present in a coordination complex include, for example, bipyridine,bipyrazine, ethylenediamine, diols (including ethylene glycol), and thelike. Examples of tridentate ligands that can be present a coordinationcomplex include, for example, terpyridine, diethylenetriamine,triazacyclononane, tris(hydroxymethyl)aminomethane, and the like.

The active materials in a flow battery can be disposed in an aqueouselectrolyte solution in which the active material is dissolved. As usedherein, the term “aqueous electrolyte solution” refers to a homogeneousliquid phase with water as a predominant solvent in which an activematerial is at least partially solubilized, ideally fully solubilized.This definition encompasses both solutions in water and solutionscontaining a water-miscible organic solvent as a minority component ofan aqueous phase.

Illustrative water-miscible organic solvents that can be present in anaqueous electrolyte solution include, for example, alcohols and glycols,optionally in the presence of one or more surfactants or othercomponents discussed below. In more specific embodiments, an aqueouselectrolyte solution can contain at least about 98% water by weight. Inother more specific embodiments, an aqueous electrolyte solution cancontain at least about 55% water by weight, or at least about 60% waterby weight, or at least about 65% water by weight, or at least about 70%water by weight, or at least about 75% water by weight, or at leastabout 80% water by weight, or at least about 85% water by weight, or atleast about 90% water by weight, or at least about 95% water by weight.In some embodiments, an aqueous electrolyte solution can be free ofwater-miscible organic solvents and consist of water alone as a solvent.

In further embodiments, an aqueous electrolyte solution can include aviscosity modifier, a wetting agent, or any combination thereof.Suitable viscosity modifiers can include, for example, corn starch, cornsyrup, gelatin, glycerol, guar gum, pectin, and the like. Other suitableexamples will be familiar to one having ordinary skill in the art.Suitable wetting agents can include, for example, various non-ionicsurfactants and/or detergents. In some or other embodiments, an aqueouselectrolyte solution can further include a glycol or a polyol. Suitableglycols can include, for example, ethylene glycol, diethylene glycol,and polyethylene glycol. Suitable polyols can include, for example,glycerol, mannitol, sorbitol, pentaerythritol, andtris(hydroxymethyl)aminomethane. Inclusion of any of these components inan aqueous electrolyte solution can help promote dissolution of acoordination complex or similar active material and/or reduce viscosityof the aqueous electrolyte solution for conveyance through a flowbattery, for example.

In addition to a solvent and a coordination complex as an activematerial, an aqueous electrolyte solution can also include one or moremobile ions (i.e., an extraneous electrolyte). In some embodiments,suitable mobile ions can include proton, hydronium, or hydroxide. Inother various embodiments, mobile ions other than proton, hydronium, orhydroxide can be present, either alone or in combination with proton,hydronium or hydroxide. Such alternative mobile ions can include, forexample, alkali metal or alkaline earth metal cations (e.g., Li⁺, Na⁺,K⁺, Mg²⁺, Ca²⁺ and Sr²⁺) and halides (e.g., F⁻, Cl⁻, or Br⁻). Othersuitable mobile ions can include, for example, ammonium andtetraalkylammonium ions, chalcogenides, phosphate, hydrogen phosphate,phosphonate, nitrate, sulfate, nitrite, sulfite, perchlorate,tetrafluoroborate, hexafluorophosphate, and any combination thereof. Insome embodiments, less than about 50% of the mobile ions can constituteprotons, hydronium, or hydroxide. In other various embodiments, lessthan about 40%, less than about 30%, less than about 20%, less thanabout 10%, less than about 5%, or less than about 2% of the mobile ionscan constitute protons, hydronium, or hydroxide.

Flow batteries can provide sustained charge or discharge cycles ofseveral hour durations. As such, they can be used to smooth energysupply/demand profiles and provide a mechanism for stabilizingintermittent power generation assets (e.g., from renewable energysources such as solar and wind energy). It should be appreciated, then,that various embodiments of the present disclosure include energystorage applications where such long charge or discharge durations aredesirable. For example, in non-limiting examples, the flow batteries ofthe present disclosure can be connected to an electrical grid to allowrenewables integration, peak load shifting, grid firming, baseload powergeneration and consumption, energy arbitrage, transmission anddistribution asset deferral, weak grid support, frequency regulation, orany combination thereof. When not connected to an electrical grid, theflow batteries of the present disclosure can be used as power sourcesfor remote camps, forward operating bases, off-grid telecommunications,remote sensors, the like, and any combination thereof. Further, whilethe disclosure herein is generally directed to flow batteries, it is tobe appreciated that other electrochemical energy storage media canincorporate the electrolyte solutions and coordination complexesdescribed herein, specifically those utilizing stationary electrolytesolutions.

In some embodiments, flow batteries can include: a first chambercontaining a negative electrode contacting a first aqueous electrolytesolution; a second chamber containing a positive electrode contacting asecond aqueous electrolyte solution, and a separator disposed betweenthe first and second electrolyte solutions. The chambers provideseparate reservoirs within the cell, through which the first and/orsecond electrolyte solutions circulate so as to contact the respectiveelectrodes and the separator. Each chamber and its associated electrodeand electrolyte solution define a corresponding half cell. The separatorprovides several functions which include, for example, (1) serving as abarrier to mixing of the first and second electrolyte solutions, (2)electrically insulating to reduce or prevent short circuits between thepositive and negative electrodes, and (3) to facilitate ion transportbetween the positive and negative electrolyte chambers, therebybalancing electron transport during charge and discharge cycles. Thenegative and positive electrodes provide a surface where electrochemicalreactions can take place during charge and discharge cycles. During acharge or discharge cycle, electrolyte solutions can be transported fromseparate storage tanks through the corresponding chambers, as shown inFIG. 1. In a charging cycle, electrical power can be applied to the cellsuch that the active material contained in the second electrolytesolution undergoes a one or more electron oxidation and the activematerial in the first electrolyte solution undergoes a one or moreelectron reduction. Similarly, in a discharge cycle the second activematerial is reduced and the first active material is oxidized togenerate electrical power.

The separator can be a porous membrane in some embodiments and/or anionomer membrane in other various embodiments. In some embodiments, theseparator can be formed from an ionically conductive polymer. Regardlessof its type, the separator or membrane can be ionically conductivetoward various ions.

Polymer membranes can be anion—or cation-conducting electrolytes. Wheredescribed as an “ionomer,” the term refers to polymer membranecontaining both electrically neutral repeating units and ionizedrepeating units, where the ionized repeating units are pendant andcovalently bonded to the polymer backbone. In general, the fraction ofionized units can range from about 1 mole percent to about 90 molepercent. For example, in some embodiments, the content of ionized unitsis less than about 15 mole percent; and in other embodiments, the ioniccontent is higher, such as greater than about 80 mole percent. In stillother embodiments, the ionic content is defined by an intermediaterange, for example, in a range of about 15 to about 80 mole percent.Ionized repeating units in an ionomer can include anionic functionalgroups such as sulfonate, carboxylate, and the like. These functionalgroups can be charge balanced by, mono-, di-, or higher-valent cations,such as alkali or alkaline earth metals. Ionomers can also includepolymer compositions containing attached or embedded quaternaryammonium, sulfonium, phosphazenium, and guanidinium residues or salts.Suitable examples will be familiar to one having ordinary skill in theart.

In some embodiments, polymers useful as a separator can include highlyfluorinated or perfluorinated polymer backbones. Certain polymers usefulin the present disclosure can include copolymers of tetrafluoroethyleneand one or more fluorinated, acid-functional co-monomers, which arecommercially available as NAFION™ perfluorinated polymer electrolytesfrom DuPont. Other useful perfluorinated polymers can include copolymersof tetrafluoroethylene and FSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂, FLEMION™ andSELEMION™.

Additionally, substantially non-fluorinated membranes that are modifiedwith sulfonic acid groups (or cation exchanged sulfonate groups) canalso be used. Such membranes can include those with substantiallyaromatic backbones such as, for example, polystyrene, polyphenylene,biphenyl sulfone (BPSH), or thermoplastics such as polyetherketones andpolyethersulfones.

Battery-separator style porous membranes, can also be used as theseparator. Because they contain no inherent ionic conductioncapabilities, such membranes are typically impregnated with additives inorder to function. These membranes typically contain a mixture of apolymer and inorganic filler, and open porosity. Suitable polymers caninclude, for example, high density polyethylene, polypropylene,polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE).Suitable inorganic fillers can include silicon carbide matrix material,titanium dioxide, silicon dioxide, zinc phosphide, and ceria.

Separators can also be formed from polyesters, poly etherketones,poly(vinyl chloride), vinyl polymers, and substituted vinyl polymers.These can be used alone or in combination with any previously describedpolymer.

Porous separators are non-conductive membranes which allow chargetransfer between two electrodes via open channels filled withelectrolyte. The permeability increases the probability of activematerials passing through the separator from one electrode to anotherand causing cross-contamination and/or reduction in cell energyefficiency. The degree of this cross-contamination can depend on, amongother features, the size (the effective diameter and channel length),and character (hydrophobicity/hydrophilicity) of the pores, the natureof the electrolyte, and the degree of wetting between the pores and theelectrolyte.

The pore size distribution of a porous separator is generally sufficientto substantially prevent the crossover of active materials between thetwo electrolyte solutions. Suitable porous membranes can have an averagepore size distribution of between about 0.001 nm and 20 micrometers,more typically between about 0.001 nm and 100 nm. The size distributionof the pores in the porous membrane can be substantial. In other words,a porous membrane can contain a first plurality of pores with a verysmall diameter (approximately less than 1 nm and a second plurality ofpores with a very large diameter (approximately greater than 10micrometers). The larger pore sizes can lead to a higher amount ofactive material crossover. The ability for a porous membrane tosubstantially prevent the crossover of active materials can depend onthe relative difference in size between the average pore size and theactive material. For example, when the active material is a metal centerin a coordination complex, the average diameter of the coordinationcomplex can be about 50% greater than the average pore size of theporous membrane. On the other hand, if a porous membrane hassubstantially uniform pore sizes, the average diameter of thecoordination complex can be about 20% larger than the average pore sizeof the porous membrane. Likewise, the average diameter of a coordinationcomplex is increased when it is further coordinated with at least onewater molecule. The diameter of a coordination complex of at least onewater molecule is generally considered to be the hydrodynamic diameter.In such embodiments, the hydrodynamic diameter is generally at leastabout 35% greater than the average pore size. When the average pore sizeis substantially uniform, the hydrodynamic radius can be about 10%greater than the average pore size.

In some embodiments, the separator can also include reinforcementmaterials for greater stability. Suitable reinforcement materials caninclude nylon, cotton, polyesters, crystalline silica, crystallinetitania, amorphous silica, amorphous Mania, rubber, asbestos, wood orany combination thereof.

Separators within the flow batteries of the present disclosure can havea membrane thickness of less than about 500 micrometers, or less thanabout 300 micrometers, or less than about 250 micrometers, or less thanabout 200 micrometers, or less than about 100 micrometers, or less thanabout 75 micrometers, or less than about 50 micrometers, or less thanabout 30 micrometers, or less than about 25 micrometers, or less thanabout 20 micrometers, or less than about 15 micrometers, or less thanabout 10 micrometers. Suitable separators can include those in which theflow battery is capable of operating with a current efficiency ofgreater than about 85% with a current density of 100 mA/cm² when theseparator has a thickness of 100 micrometers. In further embodiments,the flow battery is capable of operating at a current efficiency ofgreater than 99.5% when the separator has a thickness of less than about50 micrometers, a current efficiency of greater than 99% when theseparator has a thickness of less than about 25 micrometers, and acurrent efficiency of greater than 98% when the separator has athickness of less than about 10 micrometers. Accordingly, suitableseparators include those in which the flow battery is capable ofoperating at a voltage efficiency of greater than 60% with a currentdensity of 100 mA/cm². in further embodiments, suitable separators caninclude those in which the flow battery is capable of operating at avoltage efficiency of greater than 70%, greater than 80% or even greaterthan 90%.

The diffusion rate of the first and second active materials through theseparator can be less than about 1×10⁻⁵ mol cm⁻² day⁻¹, or less thanabout 1×10⁻⁶ mol cm⁻² day⁻¹, or less than about 1×10⁻⁷ mol cm⁻² day⁻¹,or less than about 1×10⁻⁹mol cm⁻² day⁻¹, or less than about 1×10⁻¹¹ molcm⁻² day⁻¹, or less than about 1×10⁻¹³ mol cm⁻² day⁻¹, or less thanabout 1×10⁻¹⁵ mol cm⁻²day⁻¹.

The flow batteries can also include an external electrical circuit inelectrical communication with the first and second electrodes. Thecircuit can charge and discharge the flow battery during operation.Reference to the sign of the net ionic charge of the first, second, orboth active materials relates to the sign of the net ionic charge inboth oxidized and reduced forms of the redox active materials under theconditions of the operating flow battery. Further exemplary embodimentsof a flow battery provide that (a) the first active material has anassociated net positive or negative charge and is capable of providingan oxidized or reduced form over an electric potential in a range of thenegative operating potential of the system, such that the resultingoxidized or reduced form of the first active material has the samecharge sign (positive or negative) as the first active material and theionomer membrane also has a net ionic charge of the same sign; and (b)the second active material has an associated net positive or negativecharge and is capable of providing an oxidized or reduced form over anelectric potential in a range of the positive operating potential of thesystem, such that the resulting oxidized or reduced form of the secondactive material has the same charge sign (positive or negative sign) asthe second active material and the ionomer membrane also has a net ioniccharge of the same sign; or both (a) and (b). The matching charges ofthe first and/or second active materials and the ionomer membrane canprovide a high selectivity. More specifically, charge matching canprovide less than about 3%, less than about 2%, less than about 1%, lessthan about 0.5%, less than about 0.2%, or less than about 0.1% of themolar flux of ions passing through the ionomer membrane as beingattributable to the first or second active material. The term “molarflux of ions” refers to the amount of ions passing through the ionomermembrane, balancing the charge associated with the flow of externalelectricity/electrons. That is, the flow battery is capable of operatingor operates with the substantial exclusion of the active materials bythe ionomer membrane, and such exclusion can be promoted through chargematching.

Flow batteries of the present disclosure can have one or more of thefollowing operating characteristics: (a) where, during the operation ofthe flow battery, the first or second active materials comprise lessthan about 3% of the molar flux of ions passing through the ionomermembrane; (b) where the round trip current efficiency is greater thanabout 70%, greater than about 80%, or greater than about 90%; (c) wherethe round trip current efficiency is greater than about 90%; (d) wherethe sign of the net ionic charge of the first, second, or both activematerials is the same in both oxidized and reduced forms of the activematerials and matches that of the ionomer membrane; (e) where theionomer membrane has a thickness of less than about 100 μm, less thanabout 75 μm, less than about 50 μm, or less than about 250 μm; (f) wherethe flow battery is capable of operating at a current density of greaterthan about 100 mA/cm² with a round trip voltage efficiency of greaterthan about 60%; and (g) where the energy density of the electrolytesolutions is greater than about 10 Wh/L, greater than about 20 Wh/L, orgreater than about 30 Wh/L.

In some cases, a user may desire to provide higher charge or dischargevoltages than available from a single electrochemical cell. In suchcases, several battery cells can be connected in series such that thevoltage of each cell is additive. This forms a bipolar stack, alsoreferred to as an electrochemical stack. As discussed herein, a bipolarplate can be employed to connect adjacent electrochemical cells in abipolar stack, which allows for electron transport to take place butprevents fluid or gas transport between adjacent cells. The positiveelectrode compartments and negative electrode compartments of individualcells can be fluidically connected via common positive and negativefluid manifolds in the bipolar stack. In this way, individual cells canbe stacked in series to yield a voltage appropriate for DC applicationsor conversion to AC applications.

In additional embodiments, the cells, bipolar stacks, or batteries canbe incorporated into larger energy storage systems, suitably includingpiping and controls useful for operation of these large units. Piping,control, and other equipment suitable for such systems are known in theart, and can include, for example, piping and pumps in fluidcommunication with the respective chambers for moving electrolytesolutions into and out of the respective chambers and storage tanks forholding charged and discharged electrolytes. The cells, cell stacks, andbatteries of this disclosure can also include an operation managementsystem. The operation management system can be any suitable controllerdevice, such as a computer or microprocessor, and can contain logiccircuitry that sets operation of any of the various valves, pumps,circulation loops, and the like.

In more specific embodiments, a flow battery system can include a flowbattery (including a cell or cell stack); storage tanks and piping forcontaining and transporting the electrolyte solutions; control hardwareand software (which may include safely systems); and a powerconditioning unit. The flow battery cell stack accomplishes theconversion of charging and discharging cycles and determines the peakpower. The storage tanks contain the positive and negative activematerials, such as the coordination complexes disclosed herein, and thetank volume determines the quantity of energy stored in the system. Thecontrol software, hardware, and optional safety systems suitably includesensors, mitigation equipment and other electronic/hardware controls andsafeguards to ensure safe, autonomous, and efficient operation of theflow battery system. A power conditioning unit can be used at the frontend of the energy storage system to convert incoming and outgoing powerto a voltage and current that is optimal for the energy storage systemor the application. For the example of an energy storage systemconnected to an electrical grid, in a charging cycle the powerconditioning unit can convert incoming AC electricity into DCelectricity at an appropriate voltage and current for the cell stack. Ina discharging cycle, the stack produces DC electrical power and thepower conditioning unit converts it to AC electrical power at theappropriate voltage and frequency for grid applications.

Where not otherwise defined hereinabove or understood by one havingordinary skill in the art, the definitions in the following paragraphswill be applicable to the present disclosure.

As used herein, the term “energy density” refers to the amount of energythat can be stored, per unit volume, in the active materials. Energydensity refers to the theoretical energy density of energy storage andcan be calculated by Equation 1:Energy density=(26.8 A-h/mol)×OCV×[e ⁻]  (1)where OCV is the open circuit potential at 50% state of charge, (26.8A-h/mol) is Faraday's constant, and [e⁻] is the concentration ofelectrons stored in the active material at 99% state of charge. In thecase that the active materials largely are an atomic or molecularspecies for both the positive and negative electrolyte, [e⁻] can becalculated by Equation 2 as:[e ⁻]=[active materials]×N/2  (2)where [active materials] is the molar concentration of the activematerial in either the negative or positive electrolyte, whichever islower, and N is the number of electrons transferred per molecule ofactive material. The related term “charge density” refers to the totalamount of charge that each electrolyte contains. For a givenelectrolyte, the charge density can be calculated by Equation 3Charge density=(26.8 A-h/mol)×[active material]×N  (3)where [active material] and n are as defined above.

As used herein, the term “current density” refers to the total currentpassed in an electrochemical cell divided by the geometric area of theelectrodes of the cell and is commonly reported in units of mA/cm².

As used herein, the term “current efficiency” (I_(eff)) is the ratio ofthe total charge produced upon discharge of a cell to the total chargepassed during charging. The current efficiency can be a function of thestate of charge of the flow battery. In some non-limiting embodiments,the current efficiency can be evaluated over a state of charge range ofabout 35% to about 60%.

As used herein, the term “voltage efficiency” is the ratio of theobserved electrode potential, at a given current density, to thehalf-cell potential for that electrode (×100%). Voltage efficiencies canbe described for a battery charging step, a discharging step, or a“round trip voltage efficiency.” The round trip voltage efficiency(V_(eff,RT)) at a given current density can be calculated from the cellvoltage at discharge (V_(discharge)) and the voltage at charge(V_(charge)) using Equation 4:V _(eff,RT) =V _(discharge) /V _(charge)×100%   (4)

As used herein, the terms “negative electrode” and “positive electrode”are electrodes defined with respect to one another, such that thenegative electrode operates or is designed or intended to operate at apotential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to a reversible hydrogen electrode. The negativeelectrode is associated with a first electrolyte solution and thepositive electrode is associated with a second electrolyte solution, asdescribed herein. The electrolyte solutions associated with the negativeand positive electrodes may be described as negolytes and posolytes,respectively.

Although the disclosure has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these are only illustrative of the disclosure. It should beunderstood that various modifications can be made without departing fromthe spirit of the disclosure. The disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the disclosure. Additionally,while various embodiments of the disclosure have been described, it isto be understood that aspects of the disclosure may include only some ofthe described embodiments. Accordingly, the disclosure is not to be seenas limited by the foregoing description.

What is claimed is the following:
 1. A flow battery cell comprising: afirst half-cell containing a first electrode with a first face and asecond face that are directionally opposite one another, the second faceof the first electrode being in interfacial contact with a first bipolarplate, the first bipolar plate comprising a first set of non-overlappingflow channels that interface with the first electrode; a secondhalf-cell containing a second electrode with a first face and a secondface that are directionally opposite one another, the second face of thesecond electrode being in interfacial contact with a second bipolarplate, the second bipolar plate comprising a first set ofnon-overlapping flow channels that interface with the second electrode;and a separator disposed between the first half-cell and the secondhalf-cell; wherein the first face of both the first and secondelectrodes is disposed adjacent to the separator, and the first face ofboth the first electrode and the second electrode is more hydrophilicthan is the second face.
 2. The flow battery cell of claim 1, wherein atleast one of the first electrode and the second electrode has ahydrophilicity gradient decreasing outwardly from the separator.
 3. Theflow battery cell of claim 2, wherein the hydrophilicity gradient is astepped gradient.
 4. The flow battery cell of claim 1, wherein at leastone of the first electrode and the second electrode further comprises aconductive additive deposited upon the second face to a greater extentthan upon the first face.
 5. The flow battery cell of claim 1, whereinat least one of the first electrode and the second electrode comprises acarbon electrode.
 6. The flow battery cell of claim 5, wherein at leastone of the first electrode and the second electrode comprises a firstcarbon cloth and a second carbon cloth that are layered together suchthat the first carbon cloth is adjacent to the separator, and the firstcarbon cloth is more hydrophilic than is the second carbon cloth.
 7. Theflow battery cell of claim 5, wherein the first surface of the carbonelectrode is oxidized by plasma treatment to provide oxygenatedfunctionalities on the first face, thereby rendering the first face morehydrophilic than the second surface of the same carbon electrode.
 8. Theflow battery cell of claim 5, wherein the carbon electrode isfunctionalized upon the second face with a plurality of hydrophobicmolecules to a greater extent than upon the first face, therebyrendering the first face more hydrophilic.
 9. The flow battery cell ofclaim 5, wherein the carbon electrode is functionalized upon the firstface with a plurality of hydrophilic molecules to a greater extent thanupon the second face, thereby rendering the first face more hydrophilic.